High Pings every minute or so after Fibre upgrade

Okay, so I am with Zen and using their Fritzbox 7530. I have been with them for a few years on Broadband (same router) and have had zero issues.

I have since upgraded to FTTP (small business) and I am having serious issues with the internal wifi network. I never noticed this before the upgrade, so this is something to do with FTTP, but my latency periodically spikes to nearly 1s (yes, 1000ms) and occasionally disconnects when I am on a MS Teams call (don't judge, this is due to the client using Teams).

It was particularly bad today.

I have the Fritzbox as the modem and I recently purchased a Netgear EAX20 extender so that I can connect the work and personal computers through the ethernet ports rather than trying to overload the wifi with multiple connections. Ethernet or powerline adapters are not an option.

I thought this had solved the issue, until today. The connection was very bad today, with 30+ disconnections in 1 hour!

Has anyone else had this sort of issue after upgrading to FTTP? The fibre connection is rock solid. It just seems to be the router that is cutting out the wifi everyu minute or so.